Method of finishing galvanized wire

ABSTRACT

Ferrous strand having a cleaned surface is immersed in coating bath of molten zinc having less than 0.25 percent lead and aluminum in the range of 0.04 to 0.15 percent, and withdrawing the strand therefrom in a nonoxidizing atmosphere without mechanically contacting the molten coating metal adhering to the strand. The nonoxidizing atmosphere is maintained in a tube at a temperature in the range of 850* to 950* F. through which the strand is withdrawn from the bath with substantial clearance, the tube having one end submerged in the coating metal bath and being supplied with gas at low pressure to provide a nonoxidizing atmosphere therein.

United States Patent Primary ExaminerAlfred L. Leavitt Assistant ExaminerJ. R. Batten, Jr. Attorney-Melville, Strasser, Foster & Hofi'man ABSTRACT: Ferrous strand having a cleaned surface is immersed in coating bath of molten zinc having less than 0.25 percent lead and aluminum in the range of 0.04 to 0.15 percent, and withdrawing the strand therefrom in a nonoxidizing atmosphere without mechanically contacting the molten coating metal adhering to the strand.

The nonoxidizing atmosphere is maintained in a tube at a temperature in the range of 850 to 950 F. through which the strand is withdrawn from the bath with substantial clearance, the tube having one end submerged in the coating metal bath and being supplied with gas at low pressure to provide a nonoxidizing atmosphere therein.

PATENTED JAE 4 I972 lNVENTOR/S ATTORNEYS METHOD OF FINISHING GALVANIZED WIRE BACKGROUND OF THE INVENTION This invention relates to the continuous hot dip coating of a ferrous strand with zinc, and has as its primary objective the production of Tube 3" coated wire. (The same weight of coating is designated Type 3 or Type A by the American Iron and Steel institute, and Class 3 by the American Society for Testing and Materials.)

Metallic coated ferrous wire is often described in the art as Type 1, Type 2, or Type 3 depending upon the weight of coating metal per square foot of wire surface and the uncoated wire diameter. Type I coating requires a minimum coating weight of 0.20 to 0.40 ounces of coating metal per square foot of uncoated wire surface for wire gauges 7 to 14%; Type 2 coating requires a minimum weight in the range of 0.40 to 0.60 ounces per square foot; and Type 3 coating requires a minimum coating weight of 0.80 ounces per square foot of wire surface for wire gauges 7 through 12%, and a minimum coating weight of 0.60 ounces per square foot for 14% gauge wire.

The prior art has always found it difficult to produce Type 3 coated wire with a high degree of consistency. The preferred practice generally involves the mechanical finishing of the coating by means of a nonwettable die of the type described in US Pat. Nos. 2,914,423 and 3,060,889 in the name of Earle L. Knapp. When the die described in those patents is clean, good-quality Type 3 wire is obtained. However, accumulation of material at the face of the die rapidly causes a deterioration in the quality of the coated product, and in some cases, a reduction in the coating weight. Thus, the art as described above successfully produces prime Type 3 wire less than 50 percent of the operating time.

In addition, the manufacture of mechanical coating dies must be rather precise in terms of configuration, mounting, and clearance around the wire passing therethrough.

Keeping the foregoing comments in mind, it is a primary object of this invention to provide a method and apparatus for producing Type 3 metallic coatings on a ferrous strand with a high degree of consistency.

It is a further object of this invention to provide an apparatus which is simple in design and which will carry out the method of this invention without constant attention by the operator.

SUMMARY OF THE INVENTION The method of this invention contemplates that in accordance with conventional practice a ferrous base metal strand is so treated as to render its surface clean and receptive to the molten coating metal. According to the preferred practice of the invention, the coating bath includes primarily molten zinc, with less than a specified quantity of lead, and including a small amount of aluminum.

The strand is withdrawn from the bath of molten metal in a nonoxidizing atmosphere and without mechanically contacting or otherwise finishing the molten coating metal adhering to the strand. This nonoxidizing atmosphere is maintained at least above the top of the meniscus of the coating metal drawn above the bath surface by the moving strand. The elongate tube assists in maintaining the nonoxidizing atmosphere at the meniscus without a restricted clearance.

The apparatus according to this invention includes a tube of any material which does not react readily with molten zinc, and having an inside diameter which is substantially greater than the diameter of the base strand, so that the base strand can pass through the tube with substantial clearance. One end of this tube is maintained immmersed a short distance in the coating bath, and the tube is supplied with gas at a low enough pressure that it exerts substantially no effect on the coating metal, and yet provides a nonoxidizing atmosphere. The extent of this atmosphere will be determined by the length of the tube.

DESCRIPTION OF THE DRAWINGS.

FIG. 1 is a schematic view showing an exemplary coating apparatus according to this invention.

FIG. 2 is a cross-sectional view showing the apparatus of this invention.

FIG. 3 is a cross-sectional view along the line 33 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In general, the continuous hot dip coating of a ferrous strand includes the step of thoroughly cleaning or otherwise preparing the surface of the base metal to receive the molten coating metal. The surface preparation of the strand does not per se form a part of this invention, but it is to be understood that satisfactory surface preparation will have taken place prior to the time the ferrous strand arrives at the coating metal bath.

US. Pat. Nos. 2,1 l0,893, 2,l36,957, and 2,197,662 in the name of Sendzimir all describe a preparatory procedure now in commercial use, and contemplate that the ferrous strand be subjected successively to oxidizing and reducing heat treatments before passage into the coating metal bath.

It is well recognized in the art that alkali or other chemical cleaning, or even abrasive cleaning, may be utilized in place of the oxidizing heat treatment. The essential of these preparatory steps is that the surface of the base metal strand be sufficiently cleaned that a rapid and thorough wetting by the coating metal takes place in the bath.

Turning now to FIG. I, it will be assumed that the surface of the ferrous strand to be coated has been thoroughly cleaned in preparation to receive the molten coating metal. The strand is indicated at 10, and will be understood that a suitable protective atmosphere will be provided up until the time the strand actually enters the coating metal bath.

The bath is indicated at 12. According to the preferred embodiment of the invention, the molten metal in the bath will be composed primarily of molten zinc. However, it has been discovered that in order to carry out the practice of this invention as will be described presently, the molten zinc bath must include not more than 0.25 percent lead in order to achieve a smooth, commercially acceptable coating.

It has also been found that it is desirable to add a small quantity of aluminum to the coating metal. Under normal circumstances, aluminum should be added in the range of 0.04 to 0.15 percent, preferably 0.04 to 0.08 percent. It is believed that the quantity of aluminum in the bath has a definite effect on the weight of coating metal applied. Specifically, the coating weight varies inversely with the quantity of aluminum. That is, the less aluminum present (within the limits set forth above) the heavier the applied coating.

As the ferrous strand emerges from the molten coating metal bath, it will draw with it a quantity of molten coating metal. According to this invention, the molten coating adhering to the strand is not mechanically contacted or finished in any way.

It is very important to prevent the formation of substantial oxide on the meniscus of the coating metal drawn above the level of the bath by the moving strand. To this end, the tube' indicated generally at 14 is provided. This tube may be formed of cast iron, or any other material which does not readily react with molten zinc.

As will be explained hereinafter, the lowermost end of the tube 14 is immersed in the coating metal bath, and the interior of the tube is supplied with gas under low pressure to maintain the nonoxidizing atmosphere referred to above. In commercial operation, it is preferred that the lowermost end of the tube be immersed in the coating metal far enough to maintain a seal in spite of normal variations in bath level.

Turning now to FIGS. 2 and 3, the tube 14 includes a lower portion 16 having a bore 18. It will be readily seen that the diameter of the bore 18 is substantially greater than the diameter of the wire or strand being coated. Actual experimentation has indicated that the bore 18 must be at least two times the diameter of the wire, but not more than about five times the diameter of the wire. Within these limits, coating weights increase gradually with increasing bore size up to the upper limit just mentioned. Increasing the bore diameter beyond this limit results in a rough coating. It is believed that this roughness is due to the periodic breaking of the meniscus of the coating metal.

Thus, for US. wire gauges of 7 to 12% (0.177 to 0.099 inches diameter) the bore should vary from about threeeighths inch minimum to 1 inch maximum. Because of the normal alignment problem, it is preferred that the bore be no smaller than one-quarter inch even for the finest wire such as 16 gauge 0.0625 inches).

The upper portion of the tube includes a bore 20 which is somewhat smaller in diameter than the bore 18. It will be seen that the diameter of this bore is also substantially greater than the diameter of the wire being coated. It has been found advantageous to neck down the upper portion of the bore to assist in maintaining the nonoxidizing atmosphere mentioned earlier. Once again, however, the smaller bore 20 must be large enough that there is no contact between the coated wire and the tube itself even with normal alignment problems. It is to be preferred that the smallest inside diameter of the tube be on the order of two times the diameter of the coated wire, and not smaller than one-quarter inch.

It will also be noted that the upper portion of the tube 14 is provided with the annular groove 22 which acts in effect as a plenum chamber for the nonoxidizing gas. Communication between the groove and the interior of the tube is provided by a plurality of passages 24 which are radially spaced about the tube. The drawing illustrates four such passages equally spaced about the tube and angled downwardly at an angle of approximately 45.

It is to be preferred that these passages be angled downwardly somewhat in order to insure that the gas introduced therethrough will fill the inside of the tube to positively maintain the nonoxidizing atmosphere.

The tube 14 is held in the holder 26 by means of the retaining ring 28. As indicated in FIG. 2, the holder communicates with a suitable supply of nonoxidizing gas via the pipe 30. Gas in the pipe 30 goes through the passage 32 in the holder into the groove 22 described above, and from there into the interior of the tube via the passages 24.

In a commercial operation according to the method and apparatus described above, natural gas is supplied to the pipe 30 at room temperature and at a relatively low pressure. The pressure is on the order of one-quarter ounce per square foot so it has no effect on displacing the coating. It is preferred that the gas be ignited, and the above pressure will be sufficient to burn a flame approximately 2 inches above the top of the tube 14.

A series of commercial runs were made of 9-gauge (0.148 inches) wire with the wire speed maintained at approximately 65 feet per minute. When the bore diameter of the tube was three-eighths inch, a smooth, 0.90 ounce coating was produced. Increasing the bore to one-half inch produced a smooth 1.16-ounce coating. Increasing the bore diameter to three-quarter inch produced a LOS-ounce coating which was acceptable. Increasing the bore diameter to 1 inch produced a 1.10-ounce coating which was rough and unacceptable. In the commercial embodiment referred to above, it has been found that a tube length on the order of 2% inches is entirely satisfactory for coating wire up to 14 gauge.

With the method and apparatus described, the primary factors which affect coating weight are the speed of wire passage through the bath, the temperature of the wire upon entering the bath, and the diameter of the tube. Generally speaking, it has been found that the greater the wire speed, the heavier the coating.

With respect to wire temperature, it has been found that the wire must enter the bath at a temperature in the range of 850 to 950 F. The preferred range 15 850 to 900 F. It has been found that at temperatures below 850 F, the coating is so rough as to be not commercially acceptable. Above 950 F., it is difficult to maintain minimum coating weights.

It is believed that the foregoing constitutes a full and complete disclosure of the invention. The skilled worker will appreciate that numerous modifications can be made without departing from the scope and spirit of the invention, and hence no limitations are intended except as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the continuous hot dip coating of a ferrous base strand with a coating metal wherein the surface of said strand is thoroughly cleaned prior to immersion in the molten coating metal, the steps of:

a. providing a bath of molten coating metal consisting predominantly of zinc;

b. passing said strand having an inside diameter between two and five times the diameter of the strand into said bath;

c. withdrawing said strand from said bath through an enclosure at least partially submerged in said bath, the inside diameter of said enclosure being at least on the order of two times the diameter of said strand and not more than about five times the diameter of said strand whereby to prevent mechanical contacting of the coating metal adhering to the surface thereof; and

(1. providing a nonoxidizing atmosphere for said enclosure.

2. The method claimed in claim 1 wherein said bath comprises primarily molten zinc and includes less than 0.25 percent lead.

3. The method claimed in claim 1 wherein said bath comprises primarily molten zinc and includes aluminum in the range of 0.04 to 0.15 percent.

4. The method claimed in claim 1 wherein said nonoxidizing atmosphere comprises burning natural gas.

UNITED STATES PATENT oFFlcE CERTIFICATE OF CCECTIGN Patent No. 3,632, lll Dated January l 1972 Inventor(s) MARVIN STARK It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the ABSTRACT, line 10, after "tube" insert having an inside diameter between two and five times the diameter of the strand Column 1, line 6, please delete "Tube 3" and insert Type 3 Column 3, line 15, following the word "gauge" insert In Claim 1, column t, lines 37,38, please delete "having an inside diameter between two and five times the diameter of the strand" and insert at a temperature in the range of 850 F. to 950 F.

Signed and sealed this 13th day of June 1972,

(SEAL) Attest:

EDWARD MQFLETCHER,JRQ ROBERT GOTTSCHALK Attesting Officer Commi qqi arm-rnf 'Pn'l-m )RM PO-10 USCOMM-DC 60376-P69 9 (1.5. GOVERNMENT PRINTING OFFICE: 1959 0-356-334 

2. The method claimed in claim 1 wherein said bath comprises primarily molten zinc and includes less than 0.25 percent lead.
 3. The method claimed in claim 1 wherein said bath comprises primarily molten zinc and includes aluminum in the range of 0.04 to 0.15 percent.
 4. The method claimed in claim 1 wherein said nonoxidizing atmosphere comprises burning natural gas. 